Optical communications systems as presently contemplated use an optical fiber to optically couple a light source and a photodetector. To optimize the information carrying potential of such a system, a single frequency light source is desirable as such a source minimizes problems due to, for example, chromatic dispersion of the fiber. The term "single frequency" means the spectral distribution from a light source emitting radiation in a single longitudinal mode. Such a source is also desirable for a wavelength division multiplexing system. Such single frequency light sources have been developed and one, a cleaved-coupled cavity (C.sup.3) laser, has demonstrated error-free unrepeatered transmission over a distance 119 km of optical fiber at a bit rate in excess of 400 Mbit/sec. The narrow spectral output of the cleaved-coupled cavity laser minimizes or even eliminates problems that might arise because of the dispersion characteristics of the fiber and makes such lasers ideal light sources for optical communications system that may be, for example, wavelength division multiplexing or multilevel frequency shift keying systems as well as high capacity.
The C.sup.3 lasers comprise, for example, two semiconductor diode sections. In addition to being a single frequency light source, a cleaved-coupled cavity laser may also operate as a frequency tunable light source. Frequency tuning is obtained by varying the current through one of the diode sections, termed the modulator or control section, and, consequently, the lasing wavelength of the device can be electronically controlled to select one frequency from a set of discrete frequencies.
A light source installed in an optimally designed optical communications system should be capable of operating unattended for extended time periods while maintaining stable operation as the source ages and experiences environmental changes. As a result, high degrees of reliability and stability are demanded of the light source. The stability requirements are even more stringent for single frequency, i.e., single longitudinal mode, light sources of wavelength division multiplexing systems where, in addition to the required amplitude stability, the transmitter must operate with a high degree of frequency stability for satisfactory system operation. Other optical communications systems may also require a similarly high degree of frequency stability. Even for nonwavelength division multiplexing systems, single frequency operation is frequently desirable because of the dispersion characteristics of the optical fibers.
The desirability of a frequency stabilization scheme for a C.sup.3 laser in some optical communications systems will be apparent from the following discussion. The C.sup.3 laser used in the system has two optically coupled but electrically isolated diode sections. One section is typically operated above lasing threshold and the optical output from the laser is taken from this section. The other section, conveniently termed the "modulator" or "control" section, can operate either above or below lasing threshold. When the modulator section operates below the lasing threshold, the lasing wavelength is controlled, i.e., varied, by varying the current through the modulator section. The frequency tuning results from a shift in the Fabry-Perot modes of the modulator section caused by the refractive index change which results from a variation in the carrier density as the modulator current varies. When the modulator section is operated below threshold, it is apparent that wavelength stability will often be desirable in, for example, frequency shift keying and wavelength division multiplexing systems, so that the selected frequencies remain constant. Even above threshold, a stabilization scheme may be desirable as there may be incomplete clamping of the Fermi level and, consequently, there may be some frequency shifting after the current through the modulator section is further increased after the threshold current has been reached.
Although such cleaved-coupled cavity lasers have operated with a high degree of frequency stability, it would be desirable to have a transmitter that included a stabilization scheme that was able to compensate for, for example, environmental fluctuations or laser operating changes caused by normal aging of the laser diode sections, and to maintain the same single frequency output.